(1) Field of the Invention
The present invention relates to semiconductor light-emitting devices, and more particularly relates to gallium nitride based semiconductor light-emitting devices, such as light-emitting diodes and laser diodes.
(2) Description of Related Art
In recent years, group III-V nitride semiconductors represented by a general formula, AlxGa1−x−yInyN (wherein 0≦x≦1, 0≦y≦1 and x+y≦1), have been frequently used as semiconductor materials for light-emitting devices operating in the range from visible to ultraviolet wavelengths and electronic devices operating at high powers and high temperatures.
In general, light-emitting devices using group III-V nitride semiconductors are each formed of an n-type semiconductor layer and a p-type semiconductor layer both formed on a substrate of sapphire and an active layer interposed between the n-type semiconductor layer and the p-type semiconductor layer (see, for example, Japanese Unexamined Patent Publication No. 8-70139).
In a case where a group III-V nitride semiconductor layer is formed on a substrate of sapphire, the respective lattices of the sapphire and the group III-V nitride semiconductor cannot be completely matched. The reason for this is that sapphire has a different lattice constant and thermal expansion coefficient from a group III-V nitride semiconductor. In view of the above, the number of defects in a semiconductor layer is increased, resulting in a reduction in the activation ratio of dopants, in particular, in a p-type cladding layer made of p-type aluminum gallium nitride (AlGaN) and a p-type contact layer made of p-type gallium nitride (GaN). This leads to an increase in the electrical resistances of these layers, resulting in an increase in the operating voltage of a semiconductor light-emitting device.
A method has also been considered in which the lattice mismatch between the substrate and the n-type semiconductor layer is reduced by allowing the n-type semiconductor layer to include an intermediate layer of gallium indium nitride (GaInN) or AlGaN. However, this method does not sufficiently achieve a reduction in the operating voltage of a semiconductor light-emitting device.